Linked Servers - SQL Server

Linked Servers is a feature in Microsoft SQL Server that allows you to connect to another database server and perform queries against it as if it were a local database. Linked Servers can be created for various data sources, including other SQL Server instances, Oracle databases, MySQL databases, and even Excel spreadsheets.

A linked server consists of several components: 

Local server: The SQL Server instance where the linked server is created.

Remote server: The data source that you want to access. This can be another SQL Server instance, or a different database platform.

OLE DB provider: This is the software that allows SQL Server to communicate with the remote server. SQL Server supports several OLE DB providers, such as Microsoft OLE DB Provider for SQL Server, Microsoft OLE DB Provider for Oracle, and others.

Linked server object: This is an object in SQL Server that defines the linked server and its properties, such as the name of the remote server, the OLE DB provider to use, and security options.

Security context: Linked servers can be accessed by users with different security contexts, depending on how the linked server is configured. For example, you can use a security context that maps a remote login to a local login, or you can use a fixed security context that always uses a specific set of credentials to access the linked server.

The syntax for querying data from a linked server is as follows:

SELECT * FROM linked_server_name.database_name.schema_name.table_name;

To create a linked server, follow these steps:

1) Open SQL Server Management Studio and connect to the SQL Server instance where you want to create the linked server.
2) In the Object Explorer, expand the Server Objects node, right-click on Linked Servers, and select "New Linked Server".
3) In the "New Linked Server" dialog box, enter a name for the linked server in the "Linked server" field.
4) Select the "SQL Server" option in the "Server type" field.
5) Enter the name of the remote SQL Server instance in the "Server name" field.
6) Choose the appropriate security context under "Security".
7) Go to the "Server Options" tab to configure other options like RPC, data access, and collation.

Once the linked server is created, you can query the remote database by prefixing the linked server name before the object name in your SQL statements.

Here's an example to illustrate how to use Linked Servers in SQL Server:

Suppose you have a SQL Server instance running on a computer named "SQLServerA", and you want to access data from a table named "Sales" in a database named "SalesDB" on a different SQL Server instance running on a computer named "SQLServerB". To do this, you can create a linked server on "SQLServerA" that connects to "SQLServerB".

To create a linked server on "SQLServerA" that connects to "SQLServerB", you can use the following T-SQL script:

EXEC sp_addlinkedserver
   @server='LinkedServerName',
   @srvproduct='',
   @provider='SQLNCLI',
   @datasrc='SQLServerB',
   @location='',
   @provstr='',
   @catalog='SalesDB';

After the linked server is created, you can query data from the "Sales" table in the "SalesDB" database on "SQLServerB" by using the following T-SQL statement:

SELECT * FROM LinkedServerName.SalesDB.dbo.Sales;

Note that the linked server name is used as a prefix to the database, schema, and table names in the query. The "dbo" schema is used by default, but you can specify a different schema if necessary.

The following illustration shows the basics of a linked server configuration.



Note - The SQL Server Native Client (often abbreviated SNAC) has been removed from SQL Server 2022 (16.x) and SQL Server Management Studio 19 (SSMS). Both the SQL Server Native Client OLE DB provider (SQLNCLI or SQLNCLI11) and the legacy Microsoft OLE DB Provider for SQL Server (SQLOLEDB) are not recommended for new development. Switch to the new Microsoft OLE DB Driver (MSOLEDBSQL) for SQL Server going forward.

Linked Servers provide a convenient way to access data from remote database servers within SQL Server. However, it's important to carefully consider the security implications before creating a linked server.




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